1. Field of the Invention
The present invention relates to a projector that has an electronic blackboard function and to a projector accessory that adds an electronic blackboard function to a projector.
2. Description of the Related Art
In recent years, coordinate input devices have been developed that combine a signal processor having two ultrasonic receivers and one infrared light photodetector with an electronic pen having an infrared light emitting device and an ultrasonic generator. Such devices are a form of electronic blackboard capability in which a signal processor is installed in, for example, a white board, the position of an electronic pen is obtained by measuring the distance from the electronic pen to two ultrasonic receivers, and this position then taken into a personal computer as coordinates.
Regarding the construction, a signal processor is installed in the corner of a white board, the image of a personal computer is projected by a projector onto the white board, and the position of the projected image is read into the signal processor by using an electronic pen to designate specific positions of the projected image (for example, the four corners of the board). The coordinates of the electronic pen on the board are subsequently calculated by comparing the position of the electronic pen and the previously memorized specific positions. These calculated coordinates are transferred to the personal computer, whereby figures that are drawn on the board or the movement of a mouse cursor are processed (For example, refer to Japanese Patent Laid-Open Publication No. 2002-331796 (pp. 3-5, FIG. 3)).
FIG. 1A and FIG. 1B shows a typical example of the prior art, which is next described.
As shown in FIG. 1A, signal processor 102 that is installed on screen 100 is provided with infrared photodetector 120, ultrasonic receiver 121, and ultrasonic receiver 122. Infrared light pulse 104 and ultrasonic pulse 105 (“pulse” is here used to indicate a signal that is emitted for only an instant) are simultaneously emitted from electronic pen 103. Using the same principle by which the distance of lightning can be calculated from the difference between the time the lightning is seen and the time it is heard, signal processor 102 is able to find the distance from electronic pen 103 to each of ultrasonic receivers 121 and 122 by measuring the time interval from the input of infrared light pulse 104 to infrared photodetector 120 to the input of ultrasonic waves to ultrasonic receivers 121 and 122. Ultrasonic receivers 121 and 122 are fixed to signal processor 102, and the position of electronic pen 103 as seen from ultrasonic receivers 121 and 122 can be found based on the principle of triangulation (the position of a specific point can be calculated if the distance from the specific point to two different known points is known).
As shown in FIG. 1B, an image is projected onto a whiteboard by a projector; and projected image upper left 161, projected image upper right 162, projected image lower left 163, and projected image lower right 164 are designated by electronic pen 103 to store the position of projected image 106 in signal processor 102 (initialization of coordinates). When electronic pen 103 is subsequently used within projected image 106, the position of electronic pen 103 within the image can be calculated by comparing electronic pen 103 with the position of projected image, whereby the mouse cursor of the personal computer can be moved and icons on the screen can be designated. This concludes the construction of the prior art example.
In the above-described prior-art example, however, coordinates on a planar surface can be acquired only on a whiteboard in which a signal processor has been installed, and cannot be acquired on a whiteboard that lacks a signal processor. An additional problem is the necessity for initialization of coordinates by, for example, designating the four corner of an image.